Extending battery life is one of the most important tasks faced by designers of portable electronic systems. This is particularly true for consumer electronics, such as cellular phones, digital cameras, portable computers and other handheld equipment. Designers of these products are faced with a continual need to reduce package size (and battery size) while increasing battery life to match or exceed competitive products.
Voltage regulators play an important role in extending battery life. In portable electronic systems, these regulators are used to increase, decrease and invert voltages to perform a wide range of tasks. In portable electronics, the efficiency of these devices is an important, and in some cases crucial consideration.
Switching regulators are generally considered to be among the most efficient and versatile available. For typical applications, the efficiency of switching regulators exceeds ninety percent. As shown in FIG. 1, they may be used to convert voltage upward (boost or step-up configuration). Alternately, and as shown in FIG. 2, switching regulators may be used to convert voltage downward (buck or step-down configuration). Switching regulators may even be used as shown in FIG. 3 to invert voltage (buck-boost or inverting configuration).
Switching regulators can generally be classified as either pulse-width-modulation (PWM) or pulse-frequency-modulation (PFM) types. PWM regulators produce a pulse train having a fixed frequency and a variable pulse width. PFM regulators, on the other hand, use a fixed pulse width and a variable pulse frequency. Over time, various methods have been used to enhance the basic PWM and PFM methods. For many PWM applications, this has resulted in a switch from voltage mode control to current mode control. Current mode control has a number of inherent advantages including faster response to changing loads and ease of combining the output of multiple regulators. PFM regulator topologies have been refined to support pulse skipping, hysteretic and burst mode control.
Unfortunately, neither PWM nor PFM types are without disadvantages. As shown in FIG. 4A, conventional switching regulators (a voltage mode PWM controller in this case) include a feedback loop with an analog error amplifier and comparator as core elements. As shown in FIG. 4A, the output of the PWM controller (labeled Vfb for feedback voltage) is sent to the error amplifier. The output of the error amplifier (labeled Verr for error voltage) is the difference between a reference voltage (labeled Vref) and the feedback voltage Vfb. Verr sets the threshold of a comparator whose other input is connected to a ramp voltage (Vramp). The output of the comparator drives a switch. Greater error voltages increase the comparator threshold on the comparator and increase the amount of time the switch is enabled. As the switch is held on longer, the peak current in the inductor is allowed to climb higher, storing more energy to serve the load and maintain regulation. The relationship is shown graphically in FIG. 4b. In general, this approach is characterized by low efficiency at light load, slow dynamic response with transients, and loop instability resulting from variations of component values, switching noise injection or insufficient compensational margins. In general, PFM can be used to increase efficiency for light load conditions. Unfortunately, PFM designs are prone to producing electromagnetic noise over a broad spectrum. As a result, there are many applications where PFM cannot be used.
For these reasons and others, there is a need for switching regulators that have rapid dynamic response to transients, and do not suffer from loop instability resulting from variations of component values, switching noise injection or insufficient compensational margins. This need is particularly important for applications that cannot tolerate the noise associated with PFM based regulators.